This application claims the benefit of U.S. provisional application number 60/002,807 filed on Aug. 25, 1995, now abandoned.
2. Field of the Invention
This invention resides in the field of orthopedic and dental biomaterials and it specifically relates to materials and structures designed to replace missing or diseased areas of bone. 3. Prior Art
Calcium phosphate based materials such as hydroxyapatite and .alpha. and .beta. tricalcium phosphate have been used as bone replacement materials in dental and orthopedic applications. These materials are completely biocompatible. Their bioresorbability or the rate at which they degrade and become absorbed when implanted into the body is dependent on constituent composition, crystal structure and porosity. The mineral content of bones and teeth is primarily hydroxyapatite. Hydroxyapatite is a crystalline material with the chemical formula Ca.sub.10 (PO.sub.4).sub.6 (OH).sub.2 having the Ca/P molar ratio of 1.67. Living host bone will form chemical bonds to hydroxyapatite so it has received a great deal of attention for bone replacement applications. Its synthetic form is often used to fill defects or build bone areas by allowing bone ingrowth into implanted hydroxyapatite particles and slurries. In dental applications it has been used to build up the alveolar ridge and fill areas of non-structural missing bone. Hydroxyapatite sintered from a microcrystalline powder has been used for small implants with low mechanical loading such as small bone implants for the ear. In still another application, plasma sprayed hydroxyapatite is used to coat titanium hip implants for better fixation at their interface with the femur. Thus far, however, sintered hydroxyapatite has not been used for high load bearing applications with complex stress states such as joint replacement prostheses because its current forms do not have the necessary strength and toughness to support high cyclic loads especially in tension and shear. It is also brittle and hence its strength is sensitive to internal and surface flaws.
In bone replacement applications where high strength and toughness is required, metals or combinations of metals, ceramics, and high density plastics are used. Joint replacement prostheses, for example, are fabricated from cobalt chrome alloys, titanium, stainless steel, monolithic ceramics or other relatively inert material. While they initially have sufficient strength, these materials have moduli or stiffnesses which are an order of magnitude higher than the bone they are replacing. They are also mechanically isotropic, that is, they have the same properties in all directions within the material. Bone is a low modulus, anisotropic composite material whose reinforcement is tailored to support the loads it experiences. When bone is replaced with these high modulus materials the interfacing host bone tissues are shielded from experiencing naturally occurring stress distributions. This stress shielding causes living bone tissue to resorb or become weaker and less dense. This further complicates fixation of the implant for long periods.
In joint replacement applications metal components often form a bearing surface with a plastic component on the opposite side of the joint. Wear at this bearing surface releases particles which can cause inflammatory reactions in the surrounding tissue. These problems and the continued cyclic load environment limit the long term effectiveness of the joint replacement. Hence these joint replacements are usually performed only on older patients. Joint replacement surgery in younger patients must typically be repeated after a period of time to repair damaged tissues and replace components. These added surgeries are usually higher risk and more difficult to perform.
Several U.S. patents describe applications of hydroxyapatite, mainly as a coating material for strong substrate materials. Coating methods are described in U.S. Pat. Nos. 5,279,831, 5,188,670, 5,128,169, and others. The incentive to use hydroxyapatite coatings is that the hydroxyapatite enhances the growth of dense bone in its neighborhood and further that it bonds chemically to this new bone tissue. Coating a structural material with hydroxyapatite takes advantage of these positive characteristics even though the hydroxyapatite itself, in prior art forms, does not have the strength or toughness to be used as the load bearing structure of an implant. Coatings, however, are typically very thin interfaces which are subject to flaking, chipping and the like. In addition, they only provide a two-dimensional interface upon which bone can attach limiting the strength and toughness of this attachment.
Tagai, et al in U.S. Pat. Nos. 4,735,857 and 4,820,573 describe a calcium phosphate glass fiber with a Ca/P molar ratio between 0.2 and 0.6. The use of this fiber in a cotton-like staple form or cloth form is described as a bone defect filler to encourage bone growth into the defect; however, the patents do not reveal the fiber's tensile strength or rate of resorption after implantation. A similar cotton-like product is described by Fuji in U.S. Pat. No. 4,659,617. These fiber forms may be useful in bone filling applications but in many cases the removal of diseased bone or repair of broken bones requires fixation with structures which are strong, tough and completely biocompatible. Unlike pure crystalline hydroxyapatite which has been shown to be totally biocompatible, these prior art glass fibers are comprised of mixtures of several different materials at least in part to aid in processing into fiber. These mixtures will have varying biocompatibility, resorption, and strength and hence their application to long term bone replacement components supporting high mechanical loads will be limited.